This invention relates to digital in-circuit testers for testing newly-built printed circuit assemblies. Testers usually have a central processing unit, a test-head controller, one or more driver-receiver cards, and channels. The channels are used to either drive a printed circuit assembly test node or receive a signal from that node, or both drive and receive signals. A small incircuit tester may be equipped with up to about two hundred channels, and can test small boards. Large digital in-circuit testers with more than three thousand channels also exist. Digital in-circuit testers are disclosed in the following patents: U.S. Pat. No. 4,339,819 to Jacobsen; U.S. Pat. No. 4,216,539 to Raymond et al.; U.S. Patent No. 3,943,439 to Raymond; and U.S. Pat. No. 3,931,506 to Borrelli et al. These patents are incorporated herein by reference. Related applications assigned to the Assignee of the present invention are: application Ser. No. 056,583 for "A Digital In-Circuit Tester Having a Channel-Driver Inhibitor", and application Ser. No. 055,736 for a "Digital In-Circuit Tester Having Channel-Memory Erase-Preventer. Preventer."
The assignee of the present invention makes incircuit testers which identify manufacturing defects such as wrong-value passive and non-functioning actives, including logic integrated circuits. The part of an incircuit tester that tests for correct operation of logic integrated circuits is based on Gray code and signature "analysis". However, no "analysis" is performed beyond matching of an observed signature to one previously recorded. In a digital integrated circuit test, a burst of Gray code stimuli are applied to devices' inputs and signatures are tested on the devices' outputs A "signature" is a compressed representation of a sequence of logic "highs" and "lows". The past practice has been that if an incorrect signature (i.e., one that does not match a stored value) is observed at one of the outputs of the integrated circuit, the integrated circuit is rejected. No further investigation takes place. However, users of these testers have recently stated that they would like to know which pin or pins are "open" if the rejection is due to an open pin of the surface-mounted device. An "open" pin is a pin which is not electrically connected to its corresponding node on the printed circuit board.
In the past the information as to which pin was "open" was not as important, since circuit boards were assembled from large numbers of cheap integrated circuits with through-the-hole leads. However, identification of the "open pin" has become more important as more boards are being manufactured using surface-mounted integrated circuits or other surface-mounted electronic components.
A through-the-hole integrated circuit is inexpensive to replace and usually must be replaced anyway if one of its pins is "open," since this "open" state is usually due to the pin having been crumpled during installation. In contrast, a surface-mounted integrated circuit is expensive to replace and has no pins to crumple. Thus, the "open pin" problem for a surface-mounted component can usually be solved by touching up solder on the open pin of the surface-mounted component.
The method of the present invention is particularly applicable to a digital in-circuit tester having a small central processing unit, because the method of the present invention does not require a "fault dictionary." The "fault dictionary" approach, well-known in the field of automatic test equipment, generally requires a large amount of additional memory. In contrast, the method of the present invention requires very little additional memory in the tester.